#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using mint = modint998244353;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using vmi = vector<mint>;
using vvmi = vector<vmi>;
using vvvmi = vector<vvmi>;
#define all(a) (a).begin(), (a).end()
#define rep2(i, m, n) for (int i = (m); i < (n); ++i)
#define rep(i, n) rep2(i, 0, n)
#define drep2(i, m, n) for (int i = (m)-1; i >= (n); --i)
#define drep(i, n) drep2(i, n, 0)
ll inf = 4e18;
char toc(int c){
    return c+'a';
}

int main(){
    int n; ll k; cin >> n >> k;
    string t; cin >> t;
    vvll dp(n, vll(3, inf));
    dp[n-1][0] = t[n-1]=='0' ? 0 : 1; dp[n-1][1] = 0; dp[n-1][2] = dp[n-1][0] + dp[n-1][1];
    drep(i, n-1){
        if(t[i] == '0'){
            rep(j, 3)dp[i][j] = 0;
            continue;
        }
        string u = t.substr(i, 2);
        dp[i][0] = dp[i+1][2];
        if(stoi(u) <= 26){
            if(i < n-2){
                dp[i][1] = dp[i+2][2];
            }else{
                dp[i][1] = 1;
            }
        }else{
            dp[i][1] = 0;
        }
        dp[i][2] = min(inf, dp[i][0] + dp[i][1]);
        //if(dp[i][2] >= k)break;
    }

    int itr = 0;
    while(itr < n){
        if(dp[itr][0] >= k){
            int c = t[itr] - '0';
            cout << toc(c-1);
            itr++;
        }else{
            string u = t.substr(itr, 2);
            cout << toc(stoi(u)-1);
            k -= dp[itr][0];
            itr += 2;
        }
    }cout << endl;

    return 0;
}